Wire rod for spring, steel wire for spring, spring, and method for producing same

Abstract

A spring according to one embodiment of the present invention comprises a tempered martensitic structure in which among Fe carbides precipitated in the tempered martensitic structure, carbides having a thickness of 100 nm or less have an average thickness of 5 nm to 12 nm, and the area percentage of the tempered martensitic structure may be at least 90%, and the average thickness of the carbides having a thickness of 100 nm or less is 5 nm to 12 nm. And the density of carbides having a thickness of 100 nm or less may be at least 20 particles / [mu] m2, preferably at least 40 particles / [mu] m2 and at most 64 particles / [mu] m2. Furthermore, the dislocation density of the spring according to one embodiment of the present invention may be 6.0 * 1013 / m2 to 3.0 * 1014 / m2, and the spring may contain 0.48% to 0.62% of C and 1.5% to 2.0% of Si in wt%, and further contain 0.2% to 0.55% of Mn, 0.015% or less of P, 0.020% or less of S, 0.5% to 0.8% of Cr, and 0.003% to 0.015% of N, the remainder containing Fe and unavoidable impurities.

Description

Technical Field

[0001] The present disclosure relates to a wire material for a spring, a steel wire for a spring, a spring, and a method for manufacturing the same. Background Art

[0002] Suspension springs are designed to receive repeated fatigue loads in the elastic region below the yield strength, but plastic deformation occurs in local areas, resulting in fatigue failure. In addition, due to the curb weight of the vehicle, the suspension springs are continuously subjected to compressive stress, resulting in local permanent deformation. The cause of local plastic deformation is dislocation movement caused by repeated fatigue loads and static loads occurring at stresses below the yield strength.

[0003] Generally, in order to improve fatigue strength and permanent deformation resistance of steel having a tempered martensite structure, carbide elements such as Mo, V and W are added. However, adding a large amount of relatively expensive carbide elements may reduce cost competitiveness. Therefore, it is necessary to develop a technology capable of optimizing the amount of carbide elements and a manufacturing method thereof. Summary of the invention

[0004] Technical issues

[0005] The present disclosure provides a wire material for a spring and a steel wire for a spring having improved resistance to permanent deformation, a spring, and a method for manufacturing the same.

[0006] Technical Solution

[0007] According to one embodiment of the present disclosure, the spring includes a tempered martensite structure, and among Fe carbides precipitated in the tempered martensite structure, the average thickness of carbides having a thickness of 100 nm or less is 5 nm to 12 nm.

[0008] Furthermore, the spring according to one embodiment of the present disclosure may include 90% or more of a tempered martensite structure by area fraction.

[0009] Furthermore, in the spring according to one embodiment of the present disclosure, the density of the carbide having a thickness of 100 nm or less may be 20 particles / μm. 2 or larger.

[0010] Furthermore, in the spring according to one embodiment of the present disclosure, the density of the carbide having a thickness of 100 nm or less may be 40 particles / μm. 2 or larger.

[0011] Furthermore, in the spring according to one embodiment of the present disclosure, the density of the carbide having a thickness of 100 nm or less may be 64 particles / μm. 2 or smaller.

[0012] In addition, a spring according to an embodiment of the present disclosure may have a dislocation density of 6.0×10 13 / m 2 or greater.

[0013] In addition, a spring according to an embodiment of the present disclosure may have a dislocation density of 3.0×10 14 / m 2 or less.

[0014] In addition, a spring according to an embodiment of the present disclosure may contain 0.48% to 0.62% of carbon (C) and 1.5% to 2.0% of silicon (Si) by weight percentage (wt%).

[0015] In addition, a spring according to an embodiment of the present disclosure may contain 0.2% to 0.55% of manganese (Mn), 0.015% or less of phosphorus (P), 0.020% or less of sulfur (S), 0.5% to 0.8% of chromium (Cr), 0.003% to 0.015% of nitrogen (N), the balance of iron (Fe), and unavoidable impurities by weight percentage (wt%).

[0016] In addition, in a spring according to an embodiment of the present disclosure, the average thickness of carbides having a thickness of 100 nm or less may be 6 nm to 10 nm.

[0017] In addition, the minimum outer diameter (D) of a spring according to an embodiment of the present disclosure with respect to the diameter (d) may be 4 or greater (D / d≥4).

[0018] In addition, a spring according to an embodiment of the present disclosure may have a tensile strength of 2000 MPa or greater.

[0019] In addition, in a spring according to an embodiment of the present disclosure, the permanent deformation may be 3.0 mm or less, where the permanent deformation is the height difference before and after compression for 48 hours.

[0020] A spring wire according to another embodiment of the present disclosure contains 90% or more of tempered martensite structure by area fraction, and in the Fe carbides precipitated in the tempered martensite structure, the average thickness of carbides having a thickness of 100 nm or less is 5 nm to 12 nm.

[0021] In addition, a spring wire according to an embodiment of the present disclosure may contain 0.48% to 0.62% of carbon (C) and 1.5% to 2.0% of silicon (Si) by weight percentage (wt%).

[0022] In addition, the steel wire for springs according to an embodiment of the present disclosure may contain 0.2% to 0.55% of manganese (Mn), 0.015% or less of phosphorus (P), 0.020% or less of sulfur (S), 0.5% to 0.8% of chromium (Cr), 0.003% to 0.015% of nitrogen (N), the balance of iron (Fe), and inevitable impurities, by weight percentage (wt%).

[0023] In addition, in the steel wire for springs according to an embodiment of the present disclosure, the average thickness of carbides having a thickness of 100 nm or less may be 6 nm to 10 nm.

[0024] In addition, in the steel wire for springs according to an embodiment of the present disclosure, the density of carbides having a thickness of 100 nm or less may be 40 particles / μm 2 to 64 particles / μm 2 .

[0025] In addition, the fatigue strength ratio of the steel wire for springs according to an embodiment of the present disclosure may be 0.45 or greater.

[0026] The wire rod for springs according to another embodiment of the present disclosure contains 0.48% to 0.62% of carbon (C), 1.5% to 2.0% of silicon (Si), 0.2% to 0.55% of manganese (Mn), 0.015% or less of phosphorus (P), 0.020% or less of sulfur (S), 0.5% to 0.8% of chromium (Cr), 0.003% to 0.015% of nitrogen (N), the balance of iron (Fe), and inevitable impurities, by weight percentage (wt%).

[0027] In addition, in the wire rod for springs according to an embodiment of the present disclosure, the thickness of the ferrite decarburized layer may be 0.03 mm or less.

[0028] The method for manufacturing a wire rod for springs according to another embodiment of the present disclosure includes: preparing a billet that contains 0.48% to 0.62% of carbon (C), 1.5% to 2.0% of silicon (Si), 0.2% to 0.55% of manganese (Mn), 0.015% or less of phosphorus (P), 0.020% or less of sulfur (S), 0.5% to 0.8% of chromium (Cr), 0.003% to 0.015% of nitrogen (N), the balance of iron (Fe), and inevitable impurities, by weight percentage (wt%); performing finish rolling on the billet at 930°C to 1010°C to manufacture a wire rod; and after the finish rolling, cooling the wire rod to 730°C at a cooling rate greater than 10°C / second.

[0029] A method for manufacturing spring steel wire according to another embodiment of the present disclosure includes: preparing a wire rod containing 0.48% to 0.62% carbon (C), 1.5% to 2.0% silicon (Si), 0.2% to 0.55% manganese (Mn), 0.015% or less phosphorus (P), 0.020% or less sulfur (S), 0.5% to 0.8% chromium (Cr), 0.003% to 0.015% nitrogen (N), the balance iron (Fe) and inevitable impurities by weight percentage (% by weight); reheating at 900°C to 990°C and then quenching; and tempering the quenched wire rod at 415°C to 465°C.

[0030] In addition, a method for manufacturing spring steel wire according to an embodiment of the present disclosure may further include performing secondary tempering at 300°C to 400°C after tempering.

[0031] In addition, in a method for manufacturing spring steel wire according to an embodiment of the present disclosure, the steel wire may contain 90% or more tempered martensite structure by area fraction. Among the Fe carbides precipitated in the tempered martensite structure, the average thickness of the carbides with a thickness of 100 nm or less may be 5 nm to 12 nm, and the density of the carbides may be 20 particles / μm 2 to 64 particles / μm 2 , and the dislocation density may be 6.0×10 13 / m 2 to 3.0×10 14 / m 2 .

[0032] A method for manufacturing a spring according to another embodiment of the present disclosure includes: cold-forming the manufactured compression coil spring such that the minimum outer diameter (D) of the spring is 4 or more relative to the diameter (d) of the steel wire (D / d≥4); and performing stress relief heat treatment at 190°C to 290°C.

[0033] In addition, in a method for manufacturing a spring according to an embodiment of the present disclosure, the spring may contain 90% or more tempered martensite structure by area fraction. Among the Fe carbides precipitated in the tempered martensite structure, the average thickness of the carbides with a thickness of 100 nm or less may be 5 nm to 12 nm, and the density of the carbides may be 20 particles / μm 2 to 64 particles / μm 2 , and the dislocation density may be 6.0×10 13 / m 2 to 3.0×10 14 / m 2 .

[0034] Beneficial effects

[0035] According to an embodiment of the present disclosure, by controlling alloy components and manufacturing methods, wire rods and steel wires for springs, springs, and their manufacturing methods having improved permanent deformation resistance and excellent tensile strength and fatigue strength ratio can be provided.

[0036] The technical aspects that can be achieved by the present disclosure are not limited to the above aspects, and other unmentioned technical aspects will be clearly understood by those of ordinary skill in the technical field to which the present disclosure pertains through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 A diagram showing the shape and short axis of individual carbides observed by a transmission electron microscope (TEM) in one embodiment. DETAILED DESCRIPTION

[0038] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. The following embodiments are presented to fully convey the spirit of the present disclosure to those skilled in the art to which the present disclosure pertains, and are not limited to those shown herein, but may be embodied in other forms. The drawings may omit diagrams irrelevant to the description for clarifying the present disclosure, and for illustrative purposes, the dimensions of the configuration may be enlarged.

[0039] Throughout the specification, unless otherwise specified, the term "comprising" or "including" an element does not exclude other elements, but may also include additional elements.

[0040] As used herein, unless the context clearly indicates otherwise, the singular forms are also intended to include the plural forms.

[0041] Hereinafter, a spring according to an embodiment of the present disclosure will be described.

[0042] A spring according to an embodiment of the present disclosure includes a tempered martensite structure, and among the Fe carbides precipitated in the tempered martensite structure, the average thickness of carbides having a thickness of 100 nm or less is 5 nm to 12 nm. Here, the average thickness of the carbides refers to the average value of the thicknesses of each of a plurality of individual carbides, and the thickness of the individual carbides used for calculating the average thickness refers to the length of the short axis in the cross-section of the carbide, which is not limited by the cutting direction of the cross-section of the carbide, and may be, for example, in the C-shaped cross-section or L-shaped cross-section direction. Here, the short axis may refer to the maximum width among the widths perpendicular to the long axis, and the long axis may refer to the longest diameter of the crystal grain. Figure 1FIG. 0 is a diagram showing the shape and short axis of individual carbides observed by transmission electron microscopy (TEM) in the present disclosure. Here, when measured by TEM, individual carbides generally may have an elliptical or quasi-elliptical structure, such as (a) or (b), but it is also possible to observe that they are in a shape where carbides overlap, such as (c) or (d). Referring to Figure 1 , in the case of (a) or (b), the thickness of the individual carbide refers to the short axis shown, while in the case of (c) or (d), the thickness of the individual carbide refers to the length of the longest short axis among the short axes of the carbides.

[0043] Carbides are obstacles that prevent dislocation movement. In addition, when the carbides are large, the dispersion effect is small. Therefore, in the present disclosure, carbides of 100 nm or less are defined as effective carbides because nano-carbides of 100 nm or less play a role in increasing the strength of preventing dislocation movement, and at the same time, in order to eliminate errors in the average value, micron-sized coarse carbides that have no effect on improving the resistance to permanent deformation are excluded.

[0044] In other words, the average thickness of carbides with a thickness of 100 nm or less in the tempered martensite structure is controlled to be 5 nm to 12 nm. That is to say, when the average thickness of carbides with a thickness of 100 nm or less is less than 5 nm, since dislocations pass through the carbides and move, it may not be possible to prevent the movement of dislocations, and when the average thickness exceeds 12 nm, due to the coarse carbides, the fatigue strength and resistance to permanent deformation required by the present disclosure may not be satisfied. Preferably, the average thickness of carbides with a thickness of 100 nm or less can be 6 nm to 10 nm.

[0045] When the microstructural defects serving as the strengthening mechanism of the spring and steel fail, permanent deformation may occur. The strengthening mechanism of the tempered martensite structure can be dislocations, precipitates, and solid solution elements (supersaturated [C], solid solution [Mn], etc.). Dislocations combine and annihilate with each other, precipitates combine and coarsen with each other, and supersaturated [C] precipitates, resulting in a decrease in strength and thus permanent deformation. According to the present disclosure, softening caused by dislocations in the strengthening mechanism in the environment can be prevented.

[0046] In addition, the spring according to an embodiment of the present disclosure may include 90% or more of the tempered martensite structure by area fraction.

[0047] In addition, in the spring according to an embodiment of the present disclosure, the density of carbides with a thickness of 100 nm or less can be 20 particles / μm 2 or more, preferably 40 particles / μm 2 or more, and more preferably 64 particles / μm 2or less. When the density of the fine carbide is less than 20 particles / μm 2 the movement of dislocations is free, and thus dislocations meet and annihilate, resulting in softening of the material strength. When the density is 40 particles / μm 2 the balance between strength and softening resistance is most appropriate. On the other hand, for carbides exceeding 64 particles / μm 2 the carbon content in the steel should be high. In this case, the strength increases and the formability deteriorates, so the density of carbides with a thickness of 100 nm or less is controlled to 64 particles / μm 2 or less.

[0048] In addition, the dislocation density of the spring according to an embodiment of the present disclosure can be 6.0×10 13 / m 2 or greater and 3.0×10 14 / m 2 or less. When the dislocation density exceeds 3.0×10 14 / m 2 dislocation annihilation becomes easy, and the fatigue strength and permanent deformation resistance may deteriorate. In addition, in the case of containing a carbon content of 0.48% or greater, the dislocation density can be controlled to 6.0×10 13 / m 2 or greater to achieve a strength of 2,000 MPa or greater. In particular, dislocations are formed when martensite is transformed during quenching after heating, and some of the dislocations annihilate during tempering. However, in the present disclosure, the tempering temperature can be increased by adding Si, thereby reducing the dislocation density, and the solid solution strengthening effect caused by adding Si and the precipitation strengthening effect caused by fine carbides can be controlled in a balanced manner.

[0049] In addition, the spring according to an embodiment of the present disclosure may contain 0.48% to 0.62% of carbon (C) and 1.5% to 2.0% of silicon (Si) by weight percentage (wt%), and may also contain 0.2% to 0.55% of manganese (Mn), 0.015% or less of phosphorus (P), 0.020% or less of sulfur (S), 0.5% to 0.8% of chromium (Cr), 0.003% to 0.015% of nitrogen (N), the balance of iron (Fe) and inevitable impurities.

[0050] Hereinafter, the reasons for the numerical limits of the alloy element contents in the embodiments of the present disclosure are described. Hereinafter, unless otherwise specified, the unit is wt%.

[0051] The content of C (carbon) is preferably 0.48% to 0.62%.

[0052] C is an element that effectively increases strength through solid solution strengthening, precipitation strengthening, and martensite strengthening. When the carbon content is from 0.18% to 0.70%, tempered martensite structure can be achieved. In particular, in order to ensure the formability of the spring at a strength of 2000 MPa or more required for spring products, the carbon content is preferably from 0.48% to 0.62%. In particular, an excessive C content may impair toughness, leading to sudden brittle fracture. Considering the above, the C content is preferably from 0.48% to 0.62%.

[0053] The content of Si (silicon) is preferably from 1.5% to 2.0%.

[0054] Si is an element that can be used for deoxidation of steel, suppressing the growth of carbides during tempering, and capable of increasing the tempering temperature. Si can be included in an amount of 1.2% to 2.5%. When the tempering temperature rises, due to dislocation annihilation, the dislocation strengthening effect decreases, the precipitation amount of solid solution C increases to reduce the solid solution strengthening effect, and the growth of precipitated carbides reduces the precipitation strengthening effect. In response to the strength decrease caused by the reduction of the dislocation strengthening effect and the solid solution strengthening effect of carbon, Si can delay the growth of carbides to enhance the precipitation strengthening effect, and can increase the solid solution strengthening effect of silicon to achieve the target strength. Therefore, the precipitation strengthening effect can be improved by forming ultra-fine carbides at the target strength, while dislocation annihilation may reduce the dislocation strengthening effect. Therefore, the addition of Si can minimize the annihilation of mobile dislocations during repeated fatigue, thereby increasing the resistance to permanent deformation. Considering the above, Si can be added in an amount of 1.5% or more. However, when the content of Si exceeds 2.0%, the activity of carbon increases during the addition in the wire manufacturing process, accelerating decarburization and making it difficult to control decarburization in ordinary wire manufacturing equipment. Therefore, the Si content is preferably from 1.5% to 2.0%.

[0055] The content of Mn (manganese) is preferably from 0.2% to 0.55%.

[0056] Mn is an element that improves hardenability and can be included in an amount of 1.2% or less. The addition of Mn causes S, which is an impurity in steel, to precipitate as MnS, thus preventing hot surface brittleness caused by low-melting emulsions. Considering the above, Mn can be added in an amount of 0.2% or more. However, an excessive Mn content may increase the dislocation density during quenching heat treatment and impair the resistance to permanent deformation. Therefore, the content of Mn is preferably from 0.2% to 0.55%.

[0057] The content of P (phosphorus) can be 0.015% or less.

[0058] P segregates at grain boundaries and may impair impact toughness. Considering the above, the content of P can be limited to 0.015% or less.

[0059] The content of S (sulfur) may be 0.020% or less.

[0060] Similar to P, S not only segregates at grain boundaries and reduces toughness, but is also an element that forms low-melting emulsions and hinders hot rolling. Considering the above, the content of S may be 0.020% or less.

[0061] The content of Cr (chromium) is preferably 0.5% to 0.8%.

[0062] Cr is an element effective for improving hardenability and obtaining strength, and may be included in an amount of 1.2% or less. Similar to Si, Cr plays a role in suppressing the growth of carbides during tempering. Considering the above, the content of Cr may be added in an amount of 0.5% or more. However, an excessive Cr content may form a chromium oxide layer on the surface, and the C / A ratio of corrosion pits increases, which may lead to a notch effect. Therefore, from the perspective of corrosion fatigue durability, an excessive Cr content is not preferred. Therefore, the Cr content is preferably 0.5% to 0.8%.

[0063] The content of N (nitrogen) may be 0.003% to 0.015%.

[0064] N can combine with Ti added to the steel to form nitrides. To produce an N content of less than 0.003%, the manufacturing cost may increase. However, in the case where the N content exceeds 0.015%, coarse nitrides may be formed, and the impact toughness may be reduced.

[0065] The content of V, W, Mo, Ti, or Nb may each be 0.05% or less.

[0066] V, W, Mo, Ti, or Nb form coarse carbonitrides and hinder the formation of fine carbides. Therefore, in the present disclosure, the content of each of V, W, Mo, Ti, or Nb is controlled to be 0.05% or less.

[0067] The remaining component of the present disclosure is iron (Fe). However, since during a typical manufacturing process, it may be inevitable to introduce unexpected impurities from raw materials or the surrounding environment, this may not be excluded. Since such impurities may be well-known to those skilled in the art during a typical manufacturing process, the details thereof are not described in this specification.

[0068] Furthermore, in a spring according to an embodiment of the present disclosure, the minimum outer diameter (D) relative to the diameter (d) may be 4 or greater (D / d≥4).

[0069] Furthermore, a spring according to an embodiment of the present disclosure may have a tensile strength of 2000 MPa or greater.

[0070] In addition, in a spring according to an embodiment of the present disclosure, the permanent deformation can be 3.0 mm or less, and the permanent deformation is the height difference before and after compression for 48 hours under a stress of 1274 MPa.

[0071] Hereinafter, a steel wire for a spring according to an embodiment of the present disclosure will be described.

[0072] A steel wire for a spring according to another embodiment of the present disclosure contains 90% or more of tempered martensite structure in terms of area fraction. Among the Fe carbides precipitated in the tempered martensite structure, the average thickness of the carbides with a thickness of 100 nm or less is 5 nm to 12 nm. In the tempered martensite structure, the carbides formed during tempering can restrict the movement of dislocations, thereby improving fatigue strength and resistance to permanent deformation. In addition to the tempered martensite structure, retained austenite may also be included. When the fraction of retained austenite exceeds 10%, the retained austenite transforms into martensite during spring forming, thereby reducing formability. In particular, even if no fracture occurs during forming, the martensite transformed during forming has a high dislocation density and no carbide precipitation, so dislocation annihilation is accelerated, thereby promoting the occurrence of permanent deformation. Therefore, the fraction of retained austenite is controlled to be 10% or less.

[0073] When the average thickness of the carbides with a thickness of 100 nm or less is less than 5 nm, since dislocations pass through the carbides and move, the movement of dislocations may not be blocked. When the average thickness exceeds 12 nm, due to the coarse carbides, the fatigue strength and resistance to permanent deformation required by the present disclosure may not be satisfied. Preferably, the average thickness of the carbides with a thickness of 100 nm or less can be 6 nm to 10 nm.

[0074] In addition, a steel wire for a spring according to an embodiment of the present disclosure may contain 0.48% to 0.62% of carbon (C) and 1.5% to 2.0% of silicon (Si) by weight percentage (wt%).

[0075] In addition, a steel wire for a spring according to an embodiment of the present disclosure may contain 0.2% to 0.55% of manganese (Mn), 0.015% or less of phosphorus (P), 0.020% or less of sulfur (S), 0.5% to 0.8% of chromium (Cr), 0.003% to 0.015% of nitrogen (N), the balance of iron (Fe), and inevitable impurities by weight percentage (wt%).

[0076] In addition, in a steel wire for a spring according to an embodiment of the present disclosure, the density of the carbides with a thickness of 100 nm or less can be 40 particles / μm 2 to 64 particles / μm2 .

[0077] In addition, the fatigue strength ratio (fatigue strength / tensile strength) of the wire for springs according to the present disclosure can be 0.45 or greater. When the fatigue strength ratio is less than 0.45, the tempering heat treatment temperature is lowered to increase the strength of the wire for springs and achieve the required fatigue life of the springs. As a result, the dislocation density increases and the permanent deformation resistance deteriorates.

[0078] Hereinafter, a wire for springs according to an embodiment of the present disclosure will be described.

[0079] The wire for springs according to another embodiment of the present disclosure contains, by weight percentage (wt%), 0.48% to 0.62% of carbon (C), 1.5% to 2.0% of silicon (Si), 0.2% to 0.55% of manganese (Mn), 0.015% or less of phosphorus (P), 0.020% or less of sulfur (S), 0.5% to 0.8% of chromium (Cr), 0.003% to 0.015% of nitrogen (N), the balance of iron (Fe) and inevitable impurities.

[0080] In addition, in the wire for springs according to an embodiment of the present disclosure, the thickness of the ferrite decarburized layer can be 0.03 mm or less. During the heat treatment of the wire by reheating the wire, carbon recovery occurs due to the diffusion of internal carbon to the surface caused by the austenitizing heat treatment, and the decarburized layer with a thickness of 0.03 mm or less can undergo carbon recovery. Therefore, the thickness of the ferrite decarburized layer is controlled to be 0.03 mm or less.

[0081] Next, a method for manufacturing a wire for springs according to an embodiment of the present disclosure will be described.

[0082] A method for manufacturing wire rods for springs according to another embodiment of the present disclosure includes: preparing a billet containing, by weight percentage (wt%), 0.48% to 0.62% carbon (C), 1.5% to 2.0% silicon (Si), 0.2% to 0.55% manganese (Mn), 0.015% or less phosphorus (P), 0.020% or less sulfur (S), 0.5% to 0.8% chromium (Cr), 0.003% to 0.015% nitrogen (N), the balance being iron (Fe) and inevitable impurities; performing finish rolling on the billet at 930°C to 1010°C to manufacture wire rods; and cooling the wire rods to 730°C at a cooling rate greater than 10°C / second after finish rolling. In the case where the finish rolling temperature is low, surface ferrite decarburization may occur. However, in the case where the finish rolling temperature is high, a thick total decarburized layer may appear on the surface, and the grains may coarsen, thereby preventing the target permanent deformation resistance. During the process of rapidly cooling to 730°C at 10°C / second or greater, rapid cooling is performed until the pearlite transformation region, so that the surface layer is avoided from the surface decarburization temperature region caused by the ferrite phase transformation.

[0083] Next, a method for manufacturing spring steel wires according to an embodiment of the present disclosure will be described.

[0084] A method for manufacturing spring steel wires according to another embodiment of the present disclosure includes: preparing wire rods containing, by weight percentage (wt%), 0.48% to 0.62% carbon (C), 1.5% to 2.0% silicon (Si), 0.2% to 0.55% manganese (Mn), 0.015% or less phosphorus (P), 0.020% or less sulfur (S), 0.5% to 0.8% chromium (Cr), 0.003% to 0.015% nitrogen (N), the balance being iron (Fe) and inevitable impurities; reheating at 900°C to 990°C and then quenching; and tempering the quenched wire rods at 415°C to 465°C. In the case where the reheating temperature exceeds 990°C, the grain size may become coarse. However, in the case where the reheating temperature is lower than 900°C, the pearlite may not be fully reversely transformed, and the spring formability may deteriorate due to the remaining pearlite. Therefore, the reheating temperature is controlled at 900°C to 990°C. In addition, in the case where the tempering temperature exceeds 465°C, the target tensile strength may not be satisfied, and in the case where the tempering temperature is lower than 415°C, the dislocation density is high, so the fatigue strength is reduced due to the annihilation of mobile dislocations during the fatigue test, and permanent deformation may occur. Therefore, the tempering temperature is controlled to be 415°C to 465°C.

[0085] In addition, a method for manufacturing a steel wire for springs according to an embodiment of the present disclosure may further include a secondary tempering process at 300°C to 400°C after tempering. In the case where the secondary tempering is performed at a temperature lower than 300°C, dislocation annihilation is not effectively achieved, and due to the extended heat treatment time, supersaturated carbon may selectively precipitate at the prior austenite grain boundaries, causing temper martensite embrittlement. On the other hand, in the case where the temperature exceeds 400°C, dislocation annihilation is effective, but the precipitates may coarsen, so that the desired size and density of the carbides may not be controllable.

[0086] In addition, in a method for manufacturing a steel wire for springs according to an embodiment of the present disclosure, the steel wire may include a tempered martensite structure of 90% or more by area fraction, and in the Fe carbides precipitated in the tempered martensite structure, the average thickness of the carbides with a thickness of 100 nm or less may be 5 nm to 12 nm, and the density of the carbides may be 20 particles / μm 2 to 64 particles / μm 2 , and the dislocation density may be 6.0×10 13 / m 2 to 3.0×10 14 / m 2 .

[0087] Next, a method for manufacturing a spring according to another embodiment of the present disclosure will be described.

[0088] A method for manufacturing a spring according to another embodiment of the present disclosure includes: cold-forming the manufactured compression coil spring such that the minimum outer diameter (D) of the spring product is 4 or more relative to the diameter (d) of the steel wire (D / d≥4); and performing stress relief heat treatment at 190°C to 290°C. In the case where the outer diameter of the product is less than 4, a large amount of processing occurs during cold forming, resulting in a large number of mobile dislocations. As a result, during the stress relief heat treatment for residual stress release, due to excessive dislocation annihilation, the target tensile strength may not be obtained. Therefore, in order to have a tensile strength of 2000 MPa or more, the outer diameter of the spring relative to the diameter of the steel wire should be formed to be 4 or more. In the case where the stress relief heat treatment temperature is lower than 190°C, the residual stress release is insufficient, and delayed fracture occurs at the positions where the residual hydrogen introduced during the manufacturing process is concentrated. In the case where the heat treatment temperature exceeds 290°C, dislocation annihilation and carbide growth occur, so that the target tensile strength or the target permanent deformation resistance may not be achieved.

[0089] In addition, in a method for manufacturing a spring according to an embodiment of the present disclosure, the spring may include a tempered martensite structure of 90% or more by area fraction. Among the Fe carbides precipitated in the tempered martensite structure, the average thickness of the carbides with a thickness of 100 nm or less may be 5 nm to 12 nm, the density of the carbides may be 20 particles / μm 2 to 64 particles / μm 2 , and the dislocation density may be 6.0×10 13 / m 2 to 3.0×10 14 / m 2 .

[0090] Hereinafter, the present disclosure will be described in more detail by way of embodiments. However, the description of the embodiments is only for explaining the implementation manners of the present disclosure, and the present disclosure is not limited by the description of the embodiments. This is because the scope of the rights of the present disclosure is determined by the matters described in the scope of the claims and matters reasonably inferable therefrom.

[0091] {Embodiments}

[0092] Regarding the various alloy component ranges shown in Table 1 below, ingots of 40 kg to 60 kg were manufactured in a vacuum induction melting furnace. The manufactured billets were subjected to a solution heat treatment for 4 hours in a heating furnace at 1200 °C to remove the casting structure. Thereafter, welding was performed on the rear end of the 10-meter-long billets to manufacture wire rods from the billets. The welded billets were precision rolled into wire rods with a diameter of 15 mm, rapidly cooled to 730 °C at an average cooling rate of 12 °C / second, and then slowly cooled to room temperature. Thereafter, the billets were drawn to a diameter of 13.8 mm and then reheated and quenched using an induction heating heat treatment method. The quenched wire rods were tempered to manufacture steel wires.

[0093] The precision rolling temperature, the reheating temperature, and the tempering temperature are shown in Table 2.

[0094] [Table 1]

[0095]

[0096]

[0097] [Table 2]

[0098]

[0099]

[0100] Table 3 below shows the average carbide thickness, dislocation density, tensile strength, fatigue strength ratio, and permanent deformation amount. The average carbide thickness was measured using transmission electron microscopy (TEM). Specifically, specimens prepared by the replication method were used, and 10 random positions were measured at a magnification of 160K, and then compositional analysis was performed by energy dispersive X-ray spectroscopy (EDS) to observe the average thickness of the portion confirmed to be carbide. The replicated specimens were prepared by polishing the sample with sandpaper and then etching with a 10% AA solution (acetylacetone + 1% tetramethylammonium chloride + methanol). Thereafter, carbon coating was performed on the surface of the specimen using a JEOL JEE-420 vacuum evaporator, and the specimen was mounted on a Cu grid to observe the microstructure using TEM from FEI Tecnai OSIRIS. At the same time, 3-mm thin foil specimens were prepared as follows: the sample was polished to a thickness of 80 μm with sandpaper, the specimen was punched into a 3-mm circular shape, and then electrolytic etching was performed with 10% perchloric acid and 90% acetic acid.

[0101] Meanwhile, the average carbide thickness was obtained by measuring the average thickness of the precipitated carbides with a thickness of 100 nm or less.

[0102] The density of the carbide was defined as follows: specimens prepared by the replication method and 3-mm thin foil specimens were prepared, and 10 random positions were measured at a magnification of 160,000 times each, and the area of the carbide was defined as the density. In this case, the density was measured by printing a carbide photograph, coloring the carbide on a transparent film sheet, scanning the film sheet, and then analyzing the observed area with an image analyzer (ANALYZER) program to measure the area of the carbide. The replicated specimens were prepared by polishing the sample with sandpaper and then etching with a 10% AA solution (acetylacetone + 1% tetramethylammonium chloride + methanol). Thereafter, carbon coating was performed on the surface of the specimen using a JEOL JEE-420 vacuum evaporator, and the specimen was mounted on a Cu grid to observe the microstructure using TEM from FEI Tecnai OSIRIS. At the same time, 3-mm thin foil specimens were prepared as follows: the sample was polished to a thickness of 80 μm with sandpaper, the specimen was punched into a 3-mm circular shape, and then electrolytic etching was performed with 10% perchloric acid and 90% acetic acid.

[0103] The dislocation density was measured using X-Ray Diffraction (XRD). After measurement with CuKα radiation (40 kV, 40 mA), at 40° < 2θ < 100°, and at a rate of 0.02° / second, the dislocation density was analyzed using Convolutional Multiple Whole Profile (CMWP) software. For the detailed experimental method, see K. Murasawa, Materials Transactions, Vol. 59, p. 1135 (2018).

[0104] The tensile strength was tested at a strain rate of 0.01 / second by processing a 4-mm diameter specimen in the form of a JIS No. 4 small-size specimen according to JIS Z2241.

[0105] The fatigue strength was tested according to JIS Z2274 using a JIS No. 1 specimen with a marked diameter of 5 mm by rotating bending fatigue testing in the R = -1 mode. The strength at a fatigue life of 10 million cycles was defined as the fatigue strength, and the fatigue strength ratio = fatigue strength ÷ tensile strength.

[0106] The permanent deformation was measured by compressing the specimen at 1274 MPa for 48 hours and then measuring the height difference before and after compression.

[0107] [Table 3]

[0108]

[0109]

[0110]

[0111]

[0112] Here, the TM structure fraction refers to the area fraction of tempered martensite. Referring to Table 3, Experimental Examples 1 to 22 satisfy the range of the manufacturing method and alloy components of the present disclosure. Therefore, each of Experimental Examples 1 to 22 exhibits excellent fatigue strength and permanent deformation resistance because the tempered martensite (TM) structure fraction is 90% or greater, the dislocation density is 6.0×10 13 / mm 2 to 3.0×10 14 / mm 2 , the average thickness of the carbides with a thickness of 100 nm or less in the precipitated carbides is 5 nm to 12 nm, the tensile strength is 2000 MPa or greater, the fatigue strength ratio is 0.45 or greater, and the permanent deformation is 3.0 mm or less.

[0113] However, in Experimental Example 23, the carbon (C) content was less than 0.48%, the tensile strength of 2000 MPa was not achieved, and the carbide density was only less than 20 particles / μm 2 . In addition, the fatigue strength ratio was less than 0.45, and the permanent deformation amount exceeded 3.0 mm.

[0114] In Experimental Example 24, the carbon (C) content exceeded 0.62%, so the carbide density exceeded 64 particles / μm 2 , and the average thickness of the carbides exceeded 12 nm. In addition, the dislocation density was only less than 6.0×10 13 / m 2 , the fatigue strength ratio was less than 0.45, and the permanent deformation amount exceeded 3.0 mm.

[0115] In Experimental Example 25, the silicon (Si) content was less than 1.5%, so the average thickness of the carbides exceeded 12 nm. In addition, the dislocation density was only less than 6.0×10 13 / m 2 , the tensile strength was less than 2000 MPa, the fatigue strength ratio was less than 0.45, and the permanent deformation amount exceeded 3.0 mm.

[0116] In Experimental Example 26, the silicon (Si) content exceeded 2.0%, so a decarburized layer was formed. In addition, the average carbide thickness was only less than 5 nm, and the dislocation density was less than 6.0×10 13 / m 2 , the fatigue strength ratio was less than 0.45, and the permanent deformation amount exceeded 3.0 mm.

[0117] In Experimental Example 27, the manganese (Mn) content was less than 0.2%, so the fatigue strength ratio was less than 0.45 due to MnS inclusions.

[0118] In Experimental Example 28, the manganese (Mn) content exceeded 0.55% and the dislocation density exceeded 3.0×10 14 / m 2 , so the fatigue strength ratio was less than 0.45 and the permanent deformation amount exceeded 3.0 mm.

[0119] In Experimental Example 29, the chromium (Cr) content was less than 0.5% and the dislocation density exceeded 3.0×10 14 / m 2 , so the permanent deformation amount exceeded 3.0 mm.

[0120] In Experimental Example 30, the chromium (Cr) content exceeded 0.8%, so the average carbide thickness was less than 5 nm, the carbide density exceeded 64 particles / μm 2 , the fatigue strength ratio was less than 0.45, and the permanent deformation amount exceeded 3.0 mm.

[0121] In Experimental Example 31, the finish rolling temperature was less than 930 °C, so a surface decarburized layer was formed and the fatigue strength ratio was less than 0.45.

[0122] In Experimental Example 32, the reheat temperature was less than 900 °C, and due to the remaining undissolved pearlite, the average carbide thickness exceeded 12 nm. Therefore, the fatigue strength ratio was less than 0.45 and the permanent deformation amount exceeded 3.0 mm.

[0123] In Experimental Example 33, the tempering temperature was less than 415 °C, the average carbide thickness was less than 5 nm, and the carbide density exceeded 64 particles / μm 2 , and the dislocation density also exceeded 3.0×10 14 / m 2 . Therefore, the fatigue strength ratio was less than 0.45 and the permanent deformation amount exceeded 3.0 mm. In addition, due to the low tempering temperature, the decomposition of retained austenite was insufficient, so the fraction of tempered martensite structure was also less than 90%.

[0124] In Experimental Example 34, the tempering temperature exceeded 465 °C, the average carbide thickness exceeded 12 nm, and the carbide density was less than 20 particles / μm 2 , and the dislocation density was only less than 6.0×10 13 / m 2 . Therefore, the tensile strength was less than 2000 MPa and the permanent deformation resistance also exceeded 3.0 mm.

Claims

1. A spring, the spring comprising tempered martensite structure, Among them, Among the Fe carbides precipitated in the tempered martensite structure, the average thickness of carbides with a thickness of 100 nm or less is 5 nm to 12 nm.

2. The spring according to claim 1, wherein the spring comprises 90% or more of the tempered martensite structure by area fraction.

3. The spring according to claim 1, wherein the density of the carbide having a thickness of 100 nm or less is 20 particles / μm 2 or greater.

4. The spring according to claim 1, wherein the density of the carbide having a thickness of 100 nm or less is 40 particles / μm 2 or greater.

5. The spring according to claim 1, wherein the density of the carbide having a thickness of 100 nm or less is 64 particles / μm 2 or less.

6. The spring according to claim 1, wherein the dislocation density is 6.0×10 13 / m 2 or greater.

7. The spring according to claim 1, wherein the dislocation density is 3.0×10 14 / m 2 or less.

8. The spring according to claim 1, wherein the spring contains 0.48% to 0.62% of carbon (C) and 1.5% to 2.0% of silicon (Si) by weight percentage (wt%).

9. The spring according to claim 8, wherein the spring contains 0.2% to 0.55% of manganese (Mn), 0.015% or less of phosphorus (P), 0.020% or less of sulfur (S), 0.5% to 0.8% of chromium (Cr), 0.003% to 0.015% of nitrogen (N), the balance of iron (Fe) and inevitable impurities by weight percentage (wt%).

10. The spring according to claim 1, wherein the average thickness of the carbides with a thickness of 100 nm or less is 6 nm to 10 nm.

11. The spring according to claim 1, wherein the minimum outer diameter (D) is 4 or more relative to the diameter (d) (D / d≥4).

12. The spring according to claim 1, wherein the tensile strength is 2000 MPa or more.

13. The spring according to claim 1, wherein the permanent deformation is 3.0 mm or less, and the permanent deformation is the height difference before and after compression at a stress of 1274 MPa for 48 hours.

14. A spring wire, the spring wire comprising 90% or more of the tempered martensite structure by area fraction, Among them, Among the Fe carbides precipitated in the tempered martensite structure, the average thickness of carbides with a thickness of 100 nm or less is 5 nm to 12 nm.

15. The wire according to claim 14, wherein the wire contains 0.48% to 0.62% of carbon (C) and 1.5% to 2.0% of silicon (Si) by weight percentage (wt%).

16. The wire according to claim 15, wherein the wire contains 0.2% to 0.55% of manganese (Mn), 0.015% or less of phosphorus (P), 0.020% or less of sulfur (S), 0.5% to 0.8% of chromium (Cr), 0.003% to 0.015% of nitrogen (N), the balance of iron (Fe) and inevitable impurities by weight percentage (wt%).

17. The wire according to claim 14, wherein the average thickness of the carbides with a thickness of 100 nm or less is 6 nm to 10 nm.

18. The steel wire according to claim 14, wherein the density of the carbide having a thickness of 100 nm or less is 40 particles / μm 2 to 64 particles / μm 2 .

19. The wire according to claim 14, wherein the fatigue strength ratio is 0.45 or more.

20. A wire rod for springs, the wire rod for springs containing, by weight percentage (wt%), 0.48% to 0.62% of carbon (C), 1.5% to 2.0% of silicon (Si), 0.2% to 0.55% of manganese (Mn), 0.015% or less of phosphorus (P), 0.020% or less of sulfur (S), 0.5% to 0.8% of chromium (Cr), 0.003% to 0.015% of nitrogen (N), the balance of iron (Fe) and inevitable impurities.

21. The wire rod according to claim 20, wherein the thickness of the ferrite decarburized layer is 0.03 mm or less.

22. A method for manufacturing a wire rod for springs, the method comprising: Preparing a billet, the billet containing, by weight percentage (wt%), 0.48% to 0.62% of carbon (C), 1.5% to 2.0% of silicon (Si), 0.2% to 0.55% of manganese (Mn), 0.015% or less of phosphorus (P), 0.020% or less of sulfur (S), 0.5% to 0.8% of chromium (Cr), 0.003% to 0.015% of nitrogen (N), the balance of iron (Fe) and inevitable impurities; Performing finish rolling on the billet at 930 °C to 1010 °C to manufacture the wire rod; And After the finish rolling, cooling the wire rod to 730 °C at a cooling rate greater than 10 °C / second.

23. A method for manufacturing a spring steel wire, the method comprising: Preparing a wire rod, the wire rod containing, by weight percentage (wt%), 0.48% to 0.62% of carbon (C), 1.5% to 2.0% of silicon (Si), 0.2% to 0.55% of manganese (Mn), 0.015% or less of phosphorus (P), 0.020% or less of sulfur (S), 0.5% to 0.8% of chromium (Cr), 0.003% to 0.015% of nitrogen (N), the balance of iron (Fe) and inevitable impurities; Reheating at 900 °C to 990 °C and then quenching; And Tempering the quenched wire rod at 415 °C to 465 °C.

24. The method according to claim 23, further comprising: Performing secondary tempering at 300 °C to 400 °C after the tempering.

25. The method according to claim 23, wherein the steel wire contains 90% or more of tempered martensite structure by area fraction, Among them, In the Fe carbides precipitated in the tempered martensite structure, the average thickness of the carbides with a thickness of 100 nm or less is 5 nm to 12 nm, The density of the carbide is 20 particles / μm 2 to 64 particles / μm 2 and The dislocation density is 6.0×10 13 / m 2 to 3.0×10 14 / m 2 .

26. A method for manufacturing a spring, the method comprising: Cold forming the manufactured compression helical spring such that the minimum outer diameter (D) of the spring is 4 or more relative to the diameter (d) of the steel wire manufactured by the method according to claim 23 (D / d≥4); and Performing stress relief heat treatment at 190 °C to 290 °C.

27. The method according to claim 26, wherein the spring contains 90% or more of tempered martensite structure by area fraction, Among them, Among the Fe carbides precipitated in the tempered martensite structure, the average thickness of the carbides with a thickness of 100 nm or less is 5 nm to 12 nm. The density of the carbide is 20 particles / μm 2 to 64 particles / μm 2 , and The dislocation density is 6.0×10 13 / m 2 to 3.0×10 14 / m 2 .

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